This invention pertains to microlithography in which a pattern, defined on a mask or reticle, is transferred to a suitable substrate using a charged particle beam such as an electron beam. This type of microlithography has especial utility in the fabrication of semiconductor integrated circuits and displays. More particularly, the invention pertains to reticles for charged-particle-beam microlithography and to methods for making such reticles.
In recent years, as semiconductor integrated circuits increasingly have become miniaturized, the resolution limitations of optical microlithography (i.e., microlithography performed using ultraviolet light as an energy beam) increasingly have become apparent. As a result, considerable development effort currently is being expended to develop microlithography methods and apparatus that employ an alternative type of energy beam that offers prospects of better resolution than optical microlithography. For example, considerable effort has been directed to use of X-rays. However, a practical X-ray system has not yet been developed because of many technical problems with that technology. Another candidate microlithography technology utilizes a charged particle beam, such as an electron beam or ion beam, as an energy beam.
A current type of electron-beam pattern-transfer system is an electron-beam drawing system that literally xe2x80x9cdrawsxe2x80x9d a pattern on a substrate using an electron beam. In such a system, no reticle is used. These systems can form intricate patterns having features sized at 0.1xcexcm or less because, inter alia, the electron beam itself can be focused down to a few nanometers. However, with such systems, the more intricate the pattern, the more focused the electron beam must be in order to render the pattern satisfactorily. Also, drawing a pattern line-by-line requires large amounts of time; consequently, this technology has very little utility in the mass production of semiconductor wafers where xe2x80x9cthroughputxe2x80x9d (number of wafers processed per unit time) is an important consideration.
In view of the shortcomings in electron-beam drawing systems and methods, charged-particle-beam (CPB) projection-microlithography systems have been proposed in which a reticle defining the desired pattern is irradiated with a charged particle beam. The portion of the beam passing through the irradiated region of the reticle is xe2x80x9creducedxe2x80x9d (demagnified) and projected onto a corresponding region of a wafer or other suitable substrate using a projection lens. The reticle is generally of two types. One type is a scattering-membrane reticle 21 as shown in FIG. 11(a), in which pattern features are defined by scattering bodies 24 formed on a membrane 22 that is relatively transmissive to the beam. A second type is a scattering-stencil reticle 31 as shown in FIG. 11(b), in which pattern features are defined by beam-transmissive through-holes 34 in a membrane 32 that tends to scatter particles in the beam. The membrane 32 is normally silicon with a thickness of approximately 2 xcexcm.
Because, from a practical standpoint, an entire reticle pattern cannot be projected simultaneously onto a substrate, conventional CPB microlithography reticles are divided or segmented into multiple subfields 22a, 32a each defining a respective portion of the overall pattern. The subfields 22a, 32a are separated from one another on the membrane 22, 32 by boundary regions (items 25 in FIG. 11(a)) that do not define any pattern features. In order to provide the membrane 22, 32 with sufficient mechanical strength and rigidity, support struts 23, 33 extend from the boundary regions.
Each subfield 22a, 32a typically measures approximately 1-mm square. The subfields 22a, 32a are arrayed in columns and rows across the reticle 21, 31. For projection-exposure, the subfields 22a, 32a are illuminated in a step-wise manner by the charged particle beam (serving as an xe2x80x9cillumination beamxe2x80x9d). As the illumination beam passes through each subfield, the beam becomes xe2x80x9cpatternedxe2x80x9d according to the configuration of pattern elements in the subfield. As depicted in FIG. 11(c), the patterned beam propagates through a projection-optical system (not shown) to the sensitive substrate 27. (By xe2x80x9csensitivexe2x80x9d is meant that the substrate is coated on its upstream-facing surface with a material, termed a xe2x80x9cresist,xe2x80x9d that is imprintable with an image of the pattern as projected from the reticle.) The images of the subfields have respective locations on the substrate 27 in which the images are xe2x80x9cstitchedxe2x80x9d together (i.e., situated contiguously) in the proper order to form the entire pattern on the substrate.
Referring now to FIG. 12, a conventional scattering-stencil reticle 1 is shown. The reticle 1 comprises a reticle membrane 13b, a silicon oxide layer 12a, and supporting struts 11a made of silicon. In order to increase the strength and rigidity of the reticle 1, it has been proposed to bond the reticle 1 to a support frame 4. To such end, the reticle 1 comprises a peripheral frame 11b that is essentially a wide strut extending circumferentially around the reticle. In FIG. 12, the underside of the peripheral frame 11b is bonded circumferentially to the upper surface of the support frame 4. The support frame 4 also facilitates easier handling of the reticle, especially while the reticle is being transported to and from a reticle stage in the CPB microlithography system.
The support frame 4 conventionally is made of glass such as borosilicate glass (containing mobile ions). The peripheral frame 11b, on the other hand, typically is made of silicon. One conventional manner of bonding the support frame 4 to the peripheral frame 11b is by anodic welding. The coefficient of thermal expansion of the glass in the support frame 4 is approximately 3 ppm/xc2x0 C. within the temperature range of room temperature to 500xc2x0 C., while the coefficient of thermal expansion of the silicon in the peripheral frame 11b is approximately 2 ppm/xc2x0 C. within the same temperature range. Anodic welding of silicon to glass normally is performed by applying a voltage of 500 Vdc to 1000 Vdc while heating to 300xc2x0 C. to 450xc2x0 C. Unfortunately, under such conditions, distortion of the weld joint usually occurs, due to the stated differences in coefficients of thermal expansion, as the welds return to room temperature after completion of welding. This distortion also frequently extends to the arrangement of pattern features on the reticle membrane. Membrane distortion due to welding under such conditions can be on the order of 1 xcexcm, which exceeds tolerance limits for CPB microlithography (typically 20 nm or less).
Another conventional manner of bonding the support frame to the peripheral frame of a reticle is by eutectic bonding, which is preferred from the viewpoint of preventing charging of more electrically conductive materials of the reticle. An example of such a reticle assembly 40 is shown in FIGS. 13(a)-13(b). The reticle assembly 40 comprises a reticle portion 41 bonded to a support frame 42. As shown in FIG. 13(b), the reticle portion 41 comprises a reticle membrane 46, supporting struts 47, and peripheral frame 48. Between the peripheral frame 48 and the support frame 42 is a layer 43 of silicon oxide. To perform eutectic bonding of the support frame 42 to the reticle portion 41, both the peripheral frame 48 and the support frame 42 are coated with a thin film of aluminum. The aluminum is used to create a bond in regions of contact of the support frame 42 with the peripheral frame 48. The eutectic point of aluminum-silicon is approximately 577xc2x0 C., which requires a high eutectic-bonding temperature (approximately 650xc2x0 C. to 700xc2x0 C.). Having to attain such temperatures for bonding purposes is problematic because of the time required to reach bonding temperature.
Also, this technique requires robust and costly equipment. An additional problem arises in the fact that the membrane surface of a finished reticle, as well as the reticle support, must be cleaned regularly using chlorine gas or the like to maintain cleanliness of the reticle. Exposure of eutectically bonded surfaces to chlorine gas causes corrosion of the bonds.
Another problem with conventional eutectic bonding of the support frame 42 to the peripheral frame 48 of the reticle is that, during actual welding under conditions as summarized above, thermal expansion occurs circumferentially (arrows 44) and radially (arrows 45). As a result, weld-joint distortion develops from the differences in the coefficients of thermal expansion of silicon versus the bonding metal (e.g., aluminum). This distortion causes positional distortion in the membrane 46 (especially in the patterned regions of the membrane), which causes positional distortion of the pattern features. An exemplary magnitude of distortion is approximately 200 nm, which is unacceptably high in view of the extreme accuracy (on the order of several tens of nm) currently demanded for achieving target pattern-resolution levels in microlithography.
In view of the shortcomings of the prior art as summarized above, an object of the invention is to provide CPB microlithography reticles and reticle blanks that include a support frame but in which weld-joint distortion and pattern distortion are reduced greatly compared to the prior art. Another object is to provide methods for making such reticles and reticle blanks.
To such ends, and according to a first aspect of the invention, reticles are provided for charged-particle-beam microlithography. A first exemplary embodiment of such a reticle comprises a reticle membrane, a peripheral frame, and a support frame. The reticle membrane defines features of a pattern for transfer to a sensitive substrate. The peripheral frame (desirably made of silicon) is attached peripherally to the reticle membrane. The support frame (also desirably made of silicon) is mounted to the peripheral frame via a bonding material selected from the group consisting of glasses and metals. The reticle can include struts (desirably also made of silicon and desirably contiguous with the peripheral frame) that support the membrane.
The support frame can be welded to the peripheral frame, such as circumferentially welded or spot welded. One exemplary welding method is eutectic welding involving a layer of a suitable metal (e.g., gold, aluminum, germanium, or tin) located between silicon bonding surfaces of the support frame and peripheral frame. Another exemplary welding method is anodic welding involving a layer of a suitable glass (including mobile ions) located between bonding surfaces of the support frame and peripheral frame.
The reticle membrane can be configured for use in a scattering-membrane reticle or in a scattering-stencil reticle.
A second exemplary embodiment of a reticle according to the invention comprises a reticle membrane, peripheral frame, and support frame. The reticle membrane defines features of a pattern for transfer to a sensitive substrate. The peripheral frame is attached peripherally to the reticle membrane so as to support the reticle membrane. The support frame is mounted to the peripheral frame via a glass member (wherein the glass contains mobile ions such as sodium ions). The support frame can be bonded to the peripheral frame via multiple separate glass members individually disposed around the peripheral frame between the peripheral frame and the support frame. Alternatively, the glass member can be ring-shaped between the peripheral frame and the support frame. In any event, the glass member(s) forms an anodic weld between the peripheral frame and the support frame.
In a third exemplary embodiment of a reticle according to the invention, the reticle membrane defines features of a pattern for transfer to a sensitive substrate, and a silicon peripheral frame is attached peripherally to the reticle membrane so as to support the reticle membrane. A silicon support frame is mounted to the peripheral frame via eutectic gold-silicon bonding. This reticle can further comprise a layer of chrome or nichrome situated between the eutectic gold-silicon bonding and either the support frame or the peripheral frame.
The support frame can be bonded to the peripheral frame via multiple spot welds of eutectic gold-silicon bonding individually disposed around the peripheral frame between the peripheral frame and the support frame. Also, a layer of chrome or nichrome can be situated between the eutectic gold-silicon bonding and either the support frame or the peripheral frame.
In a fourth exemplary embodiment of a reticle according to the invention, a silicon peripheral frame (including a bonding surface) is attached peripherally to the reticle membrane so as to support the reticle membrane. A silicon support frame comprising a respective bonding surface is mounted to the bonding surface of the peripheral frame via multiple eutectic metal-silicon bonding points in metallic island structures on the bonding surfaces. The metal-silicon bonding points desirably comprise a metal selected from the group consisting of gold, aluminum, germanium, and tin. The metal-silicon bonding points can further comprise chrome or nichrome layered between the island structures and either the bonding surface of the support frame or the bonding surface of the peripheral frame.
According to another aspect of the invention, reticle blanks are provided. According to a first exemplary embodiment, such a reticle blank comprises a reticle membrane, a peripheral frame peripherally attached to the reticle membrane, and a support frame. The support frame is bonded to the peripheral frame via a bonding material selected from the group consisting of glasses and metals.
In a second exemplary embodiment of a reticle blank according to the invention, a silicon peripheral frame surrounds and is attached to a reticle membrane. A silicon support frame is mounted to the peripheral frame via a glass member including mobile ions. The glass member can be configured as multiple separate glass members individually disposed around the peripheral frame between the peripheral frame and the support frame.
In a third exemplary embodiment of a reticle blank according to the invention, a silicon peripheral frame surrounds and is attached to a reticle membrane. A silicon support frame is mounted to the peripheral frame via eutectic gold-silicon bonding. Such a reticle further can comprise a layer of chrome or nichrome situated between the eutectic gold-silicon bonding and either the support frame or the peripheral frame. The support frame can be bonded to the peripheral frame via multiple spot welds of eutectic gold-silicon bonding individually disposed around the peripheral frame between the peripheral frame and the support frame.
In a fourth exemplary embodiment of a reticle blank according to the invention, a silicon peripheral frame is attached peripherally to a reticle membrane so as to support the reticle membrane. The peripheral frame comprises a respective bonding surface. The silicon support frame comprises a respective bonding surface mounted to the bonding surface of the peripheral frame via multiple eutectic metal-silicon bonding points in metallic island structures on the bonding surfaces. Desirably, the metal-silicon bonding points comprise a metal such as gold, aluminum, germanium, or tin. The metal-silicon bonding points can further comprise chrome or nichrome layered between the island structures and either the bonding surface of the support frame or the bonding surface of the peripheral frame.
According to yet another aspect of the invention, methods are provided for making a reticle for charged-particle-beam microlithography. In a first exemplary embodiment of such a method, a peripheral frame is provided that peripherally supports a reticle membrane. A support frame is provided that is configured to allow the peripheral frame to be bonded to the support frame along respective bonding surfaces. A glass member is formed on the bonding surface of at least one of the support frame and the peripheral frame. The glass member desirably includes mobile ions. The support frame and peripheral frame are placed in an intended bonding orientation such that the glass member contacts both respective bonding surfaces. The support frame is welded anodically to the peripheral frame at each glass member. Pattern features are formed on or in the reticle membrane before or after the bonding steps.
The glass member can be formed in multiple separate localized regions or in a ring configuration on the bonding surface. In the latter instance, the method can further comprise the step, before the anodic welding step, of removing portions of the glass member so as to form multiple separate localized regions on the bonding surface.
Desirably, the glass member is formed at a thickness of 1 to 50 xcexcm, thereby constituting a xe2x80x9cthinxe2x80x9d glass member. Alternatively, the glass member can be formed initially at a thickness of greater than 50 xcexcm, wherein the thickness subsequently is reduced to 1 to 50 xcexcm before the anodic welding step.
In a second exemplary embodiment of a method for making a reticle, a silicon peripheral frame is provided that peripherally supports the reticle membrane. A silicon support frame is provided that is configured to allow the peripheral frame to be bonded to the support frame along respective bonding surfaces. A metal film (e.g., gold, aluminum, germanium, or tin, desirably 200 to 2000 nm thick) is formed on the bonding surface of at least one of the support frame and the peripheral frame. The support frame and peripheral frame are placed in an intended bonding orientation such that the metal film contacts both respective bonding surfaces. A silicon-metal eutectic weld of the support frame to the peripheral frame is formed at points of contact of the support frame to the peripheral frame. The pattern features can be formed in the reticle membrane before or after the welding step.
The metal film can be formed in a ring configuration on the bonding surface. In such an instance, before forming the eutectic weld, portions of the metal film are removed so as to form multiple separate localized regions of the metal film on the bonding surface. Alternatively, the metal film is formed in multiple separate localized regions on the bonding surface.
If desired, the method can further comprises the step, before forming the eutectic weld, of forming a layer of chrome or nichrome between the metal film and the bonding surface to which the metal film is applied. The layer of chrome or nichrome desirably is formed at a thickness of 1 to 5 nm.
In a third exemplary embodiment of a method for making a reticle, a metal film as described above is formed on the bonding surface of at least one of the support frame and the peripheral frame. The support frame and peripheral frame are placed in an intended bonding orientation such that the metal film contacts both respective bonding surfaces. While so oriented, the support frame and peripheral frame are incubated at a temperature sufficient to cause the metal film to form multiple eutectic metal-silicon bonding points in metallic island structures on the bonding surfaces.
According to yet another aspect of the invention, methods are provided for making reticle blanks. In a first exemplary embodiment of such a method, a peripheral frame is provided that peripherally supports a reticle membrane. A support frame is provided that is configured to allow the peripheral frame to be bonded to the support frame along respective bonding surfaces. A glass member (the glass including mobile ions) is formed on the bonding surface of at least one of the support frame and the peripheral frame. The support frame and peripheral frame are placed in an intended bonding orientation such that the glass member contacts both respective bonding surfaces. The support frame then is welded anodically to the peripheral frame at each glass member. As noted above, the glass member can be formed in multiple separate localized regions on the bonding surface, or formed in a ring configuration on the bonding surface. In the latter instance, before the anodic welding step, portions of the glass member are removed so as to form multiple separate localized regions on the bonding surface. If the glass member is formed in a ring configuration, the glass member initially can be formed at a thickness greater than 50 xcexcm, wherein the thickness is reduced to 1 to 50 xcexcm before the anodic welding step.
In a second exemplary embodiment of a method for forming a reticle blank, a metal film (e.g., gold, aluminum, germanium, and tin) is formed on the bonding surface of at least one of the support frame and the peripheral frame. The support frame and peripheral frame are placed in an intended bonding orientation such that the metal film contacts both respective bonding surfaces. A silicon-metal eutectic weld of the support frame to the peripheral frame is formed at points of contact of the support frame to the peripheral frame. As noted above, the metal film can be formed in a ring configuration on the bonding surface, in which instance, before forming the eutectic weld, portions of the metal film can be removed so as to form multiple separate localized regions of the metal film on the bonding surface. Alternatively, the metal film can be formed in multiple separate localized regions on the bonding surface.
Before forming the eutectic weld, a layer of chrome or nichrome can be formed between the metal film and the bonding surface to which the metal film is applied. The layer of chrome or nichrome desirably is formed at a thickness of 1 to 5 nm.
In a third exemplary embodiment of a method for making a reticle blank, a metal film (as noted above) is formed on the bonding surface of at least one of the support frame and the peripheral frame. The support frame and peripheral frame are placed in an intended bonding orientation such that the metal film contacts both respective bonding surfaces. While so oriented, the support frame and peripheral frame are incubated at a temperature sufficient to cause the metal film to form multiple eutectic metal-silicon bonding points in metallic island structures on the bonding surfaces. The metal film desirably is formed in multiple separate localized regions on the bonding surface. Before forming the eutectic weld, a layer of chrome or nichrome can be formed between the metal film and the bonding surface to which the metal film is applied.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.